Perception and other cognitive functions such as planning, thought and learning reflect processing of complex information by the cerebral neocortex. The great expanse of brain tissue that comprises the cerebral cortex is composed of iterated, local neuronal circuits that transform afferent information and distribute it to other brain regions, forming large distributed neural systems. An emerging view is that these spatially distributed networks use a minimal number of spikes to perform their functions rapidly and accurately. Neuronal assemblies are readily identified in the rodent somatosensory cortex, which contains groups of synaptically interconnected neurons, called 'barrels', that a related one-to-one to individual whiskers on the contralateral face. Each barrel receives its afferent input from similarly organized groups of thalamic neurons, called 'barreloids'. We have discovered that thalamocortical circuits in the rodent somatosensory system are highly sensitive to thalamic response timing. Employing the rat whisker/barrel cortex as a model sytem, we will evaluate spike timing and response synchrony in thalamocortical circuits and their modulation by corticothalamic feedback. Hypotheses will be evaluated using microelectrodes to record simultaneously the activities of thalamic and cortical neurons that are functionally connected. The research plan is based on the premise that abnormalities in the time-critical operations of thalamocortical circuits produce a cascade of events leading to dysfunctions in cortical processing. Understanding the role of local thalamocortical circuitry in promoting adaptive properties of cerebral cortical function is essential for bridging the gap between cellular physiology and the eventual accurate diagnosis and treatment of perceptual/motor and other cognitive dysfunctions due to abnormal cortical development, aging, disease, or trauma.